carb. crazzzzyness!!!!!!!!!

There is no reason a 650 DP won't run on that car.

There is several areas of concern, and this is "carb 101"

FIRST Make sure there are no vacuum leaks. Probe around the carb base and other areas on the intake with a scrap length of fuel hose, one end to your ear. Also good for finding exhaust leaks

Check your fuel pressure. You could be pushing fuel right past the needle/ seats at too high pressure

NEXT check the float level.

Then use (water even) in a squirt bottle, short squirts around the carb base, etc. Changes in idle mean a leak

WHAT IS the timing? You want a SHORT mechanical curve and LOTS of initial advance. This alone may clear up the "carb problem"

Next, and ONLY after you get the timing set, investigate the power valve. There are lots of articles on the www, Google it. You will need to tear down the carb and read "what's in" the carb, and about all you can do to tell if it's leaking, is to REPLACE it. But you'll have to measure intake vacuum at idle (AFTER setting timing!!!) to determine if the PV is in the right range.

What you are after with the power valve, initially, is to make sure it is not coming open at idle/ low RPM.

After you have done this, get the engine fully warmed and adjust the idle speed/ idle mixture. You'll need to bounce between the two. Turn the idle mixture screws in until you feel a drop in speed, or watch a vacuum gauge, then bring them slowly out until it "peaks." Then just touch them in (CW for most) towards the lean side until the engine "not quite" drops in speed or vacuum.

Touch up the idle speed, and if you had to change it very much, re-touch the idle screws. IE on something like a stock, stick 318, you don't want to adjust the mixture at 1000RPM and then set the speed down to 600. You want to adjust the mixture as close as you can get to final idle speed.

If you can NOT get a nice idle, and you are SURE the fuel pressure, the float level, and the power valve number is OK, suspect a leaky power valve and replace it.

Next run the car and see where you are. If medium cruise is still not right, see what the plugs look like and only after all the first steps are confirmed OK, do you want to play with jet size.

Random comments.

Cold start. After the carb is correct for warm running, then you can attack the cold start. Do you run a choke? If not, the carb might be jetted on the lean side, or not have enough accelerator pump shot, and you might just expect too much. No carbureted car, example, a "smog" mid 70's car, is going to start and run decent in cold weather without fiddling with the throttle.

Does this problem get worse / better/ no change with outdoor temperature? You could be having fuel boil or vapor lock problems, or you could be having these problems ON TOP of a badly tuned carb.

WHAT KIND of DP is this? Square or spread bore? Example, years ago I had two different Holley spread bore 650's on my 340, and had the same problem with both for a short time. The rear barrels would get "sticky" and would not quite spring completely shut, which once caused the car to act as if the throttle had stuck, and also of course made for high/ erratic and poor idle. You might try a thin tie wire in the rear linkage to keep the rear barrels shut and test drive on the "2 barrel" to see if that affects the problem.